I. Field of the Invention
The present invention relates generally to a ballast circuit for a fluorescent lamp.
II. Description of the Prior Art
Fluorescent lamps typically comprise an elongated tube having an electroluminescent coating on its interior surface. An electrode is provided at each end of the tube and the electrodes are electrically connected to an alternating current power source. This alternating current causes the electrons to impinge upon the interior electroluminous coating of the lamp thus illuminating it.
In the previously known fluorescent lamp assemblies, a ballast assembly provides alternating current to the electrodes on the fluorescent lamp in order to illuminate the lamp. Typically, the ballast assembly comprises a transformer which increases the voltage from line voltage of 110-220 volts to approximately 600-800 volts. In addition, the current which flows through the lamp typically averages around 800 milliamps.
A primary disadvantage of these previously known ballast assemblies for fluorescent lamps is that a higher voltage is required for the fluorescent lamp when the fluorescent lamp is used outside in a cold environment. For example, the ballast circuit for exterior lamps in cold environments typically produces a voltage of 800 volts AC to the fluorescent lamp, while a ballast circuit for interior fluorescent lamps only produces about 600 volts AC to the lamp. Thus, two different ballast circuits are required depending upon whether the fluorescent lamp is used outside or inside.
A still further disadvantage of these previously known ballast circuits is that they provided 50 or 60 cycles per second alternating current to the fluorescent lamp electrodes. As such, they were relatively energy inefficient Such fluorescent lamps are also prone to flickering due to the relatively low frequency alternating current used to power them.
A still further disadvantage of these previously known ballast assemblies is that they are unable to variably dim the fluorescent lamp. Instead, the lamp is either fully on or fully off.